1. Field of the Invention
The present invention generally relates to a radiation detection device. More specifically, the invention relates to optical fiber-based radiation detector arrays for monitoring nuclear radiation sources.
2. Description of Related Art
There is a growing concern that terrorists, or others, may attempt to import radioactive or nuclear material which may be used for the construction of nuclear and/or radiation based weapons. Because of this concern, these materials need to be either controlled or monitored. Because of the large number of containers transported in commerce, it is difficult to thoroughly check each and every container for the presence of any type of radioactive or nuclear material.
Therefore, it is desirable to be able to detect and identify the presence of nuclear radiation sources within packaging or containment that does not permit direct visual inspection. It is desirable to be able to do so with both high detection sensitivity and the ability to locate such radiation sources within the containment or packaging when immediate physical access to the interior of the packaging is not convenient or possible. It is further desirable to do this simply, rapidly, and without the necessity of large fixed facilities since, in the case of intermodal shipping containers, it will be important to detect any contraband before the container arrives in a destination port. It is further desirable, in addition to detecting the presence of nuclear radiating materials, to locate them in the object or objects being inspected. Such a radiation detection device may also be useful in Homeland security devices and/or automobile checkpoints.
Conventional detectors, which detect nuclear radiation in the forms of alpha, beta and gamma rays, are typically expensive, have limited sensitivity, are physically fragile, have limited life, making them unsuitable for widespread field deployment. Typical issues involve aspects such as the need to have active cooling in order to achieve high enough sensitivity with solid-state detectors, high power requirements supplied at mains (high) voltages as for traditional photomultiplier tube (“PMT”) devices, and physically stable platforms on which the detectors can be mounted. Additionally it is, in many cases, necessary to collect the data and send the detection apparatus to a distant laboratory so that the read out of the data can be made in a thermally and light controlled environment not easily achieved ‘in the field.’ The time delay associated with remote processing can prevent prompt action on the results of the testing or monitoring, and that sometimes presents a serious problem.
Some radiation detectors comprise solid state materials which when interacting with gamma rays or high energy beta rays, produce electron-hole (“E-H”) pairs internally. In particular types of material, the subsequent recombination of the E-H pairs produces light output which has a photon energy characteristic both of the material and the energy of the radiation which produced the E-H pairs, and which can be detected and measured. Typical materials include optically stimulated luminescent (“OSL”) materials, such as carbon doped aluminum oxide (α-Al2O3:C), Al2O3:Cr, Mg, Fe, MgAl2O4 spinels, Mg2SiO4:Tb, and natural fluorite, europium doped flourochlorozirconate glass ceramics, or alkali impurity doped BaFBr:Eu+2 and thermal luminescent detector (“TLD”) material types. Various examples of these materials can have ranges of times and temperatures over which the separated hole-electron pairs are stable. For these materials, when in their ‘excited’ state, the recombination can be stimulated to occur rapidly either by raising their temperature (TLD) so as to speed up migration of the electrons toward their nearby holes or by exposing them to an incident light flux (OSL) which can stimulate the recombination and consequent light emission. Furthermore, there are such materials having, in the absence of external stimulation, different excited state relaxation lifetimes and whose emissions upon recombination are characteristic and identifiable by their emission wavelengths. Additionally, there exist materials which are sensitive to the energy of the nuclear radiation and whose emission wavelength also depends on that energy.
In view of the above, it is apparent that there exists a need for an easily packaged, compact radiation detector device efficient in data readout and operable for long periods on low power. There further exists a need to be able to detect and identify the presence and location of nuclear radiation sources within packaging or containment that does not permit direct visual inspection. Additionally, this detection must be done simply, rapidly, and without the necessity of large fixed facilities since, in the case of intermodal shipping containers it will be important to detect any contraband before the container arrives in a destination port. Further, monitoring boundaries of nuclear facilities or of countries for covert transit of nuclear materials is also an important need.